The present invention relates to a stable aqueous dispersion of polymer particles containing adsorbing functional groups and a process for preparing them.
Titanium dioxide (TiO2) is an expensive component in many coatings or paint formulations. The efficacy of TiO2 as a hiding pigment is reduced when TiO2 particles are allowed to come too close together upon film formation and drying (which they tend to do). It has been disclosed that the spacing of TiO2 and its resultant efficiency can be improved using an adsorbing emulsion polymer.
U.S. Pat. No. 7,179,531 (Brown et al.) discloses a dispersion of multistage polymer particles characterized by a relatively small core portion protuberating from a relatively large shell portion of each particle, with the core portion being preferentially functionalized with TiO2-adsorbing groups. These so-called “acorn” particles are disclosed as being useful for preparing TiO2-polymer composite particles that provide dried coatings with improved hiding. The preferred partitioning of adsorbing functional groups at the core portion of the acorn particles is designed to reduce bridging effects; nevertheless, “bleeding” of adsorbing groups to the shell portion of the acorn sometimes occurs, resulting in increased flocculation and concomitant grit formation. Accordingly, it would be desirable to reduce grit formation by generating acorn particles with a more effective partitioning of pigment-adsorbing functional groups at the protuberating core portion of the particles.